Syndiotactic polymers have a unique stereochemical structure in which monomeric units having enantiomorphic configuration of the asymmetrical carbon atoms follow each other alternately and regularly in the main polymer chain. Syndiotactic polypropylene was first disclosed by Natta et al. in U.S. Pat. No. 3,258,455. As disclosed in this patent, syndiotactic polypropylene is produced by using a catalyst prepared from titanium trichloride and diethyl aluminum monochloride. A later patent to Natta et al., U.S. Pat. No. 3,305,538, discloses the use of vanadium triacetylacetonate or halogenated vanadium compounds in combination with organic aluminum compounds for producing syndiotactic polypropylene. U.S. Pat. No. 3,364,190 to Emrick discloses a catalyst system composed of finely divided titanium or vanadium trichloride, aluminum chloride, a trialkyl aluminum and a phosphorus containing Lewis base as producing syndiotactic polypropylene. As disclosed in these patent references and as known in the art, the structure and properties of syndiotactic polypropylene differ significantly from those of isotactic polypropylene. The isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows: ##STR1##
Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" representing a "meso" dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure syndiotactic polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Syndiotactic polypropylene is shown in zig-zag representation as follows: ##STR2##
Corresponding representations for syndiotactic polyvinylchloride and polystyrene respectively are: ##STR3##
Using the Fischer projection formula, the structure of a syndiotactic polymer is designated as: ##STR4## In NMR nomenclature, this pentad is described as . . . rrrr . . . in which each "r" represents a "racemic" dyad, i.e., successive methyl groups on alternate sides of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and, like the isotactic polymers, are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer that is soluble in xylene. An atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product.
While it is possible for a catalyst to produce all three types of polymers, it is desirable for a catalyst to produce predominantly isotactic or syndiotactic polymer with very little atactic polymer. Catalysts that produce isotactic polyolefins are disclosed in copending U.S. patent application Ser. Nos. 034,472 filed Apr. 3, 1987 and now abandoned; 096,075 filed Sep. 11, 1987 (now U.S. Pat. No. 4,794,096); and 095,755 filed on Sep. 11, 1987 and now abandoned. These applications disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of a highly isotactic polypropylene.
Catalysts that produce syndiotactic polypropylene or other syndiotactic polyolefins are disclosed in the aforementioned application Ser. No. 220,007 (now U.S. Pat. No. 4,892,851). These catalysts are bridged stereorigid metallocene catalysts. The catalysts have a structural bridge extending between dissimilar cyclopentadienyl groups and may be characterized by the formula: EQU R"(CpR.sub.n)(CpR'.sub.m)MeQ.sub.k ( 1)
In formula (1), Cp represents a cyclopentadienyl or substituted cyclopentadienyl ring; and R and R' represent hydrocarbyl radicals having 1-20 carbon atoms R" is a structural bridge between the rings imparting stereorigidity to the catalyst; Me represents a transition metal and Q a hydrocarbyl radical or halogen. R'.sub.m is selected so that (CpR'.sub.m) is a sterically different substituted cyclopentadienyl ring than (CpR.sub.n). n varies from 0 to 4 (0 designating no hydrocarbyl groups, i.e. an unsubstituted cyclopentadienyl ring), m varies from 1-4, and K is from 0-3. The sterically different cyclopentadienyl rings produce a predominantly syndiotactic polymer rather than an isotactic polymer. Metallocene catalysts of yet another type are cationic catalysts as disclosed in European Patent Applications 277,003 to Turner et al. and 277,004 to Turner. As disclosed in these applications, a bis(cyclopentadienyl) zirconium, titanium or hafnium compound is reacted with a second compound comprising a cation capable of donating a proton or an ion exchange compound comprising a cation which will irreversible react with a ligand on the first compound, and a bulky, stable anion. The catalysts described in the European Patent Applications 277,003 and 277,004 are disclosed as especially useful in the polymerization of ethylene and more generally in the polymerization of alpha olefins, diolefins and/or an acetylenically unsaturated compounds containing from 2-18 carbon atoms. Principally disclosed in the European applications is the polymerization of ethylene or the copolymerization of ethylene with propylene or 1 butene or with propylene and 1- butene or 1,4 hexadiene. Stereospecificity, or lack thereof, of the polymers as disclosed in the Turner and Turner et al. applications are not generally discussed, although in application Ser. No. 277,004 examples are given of producing atactic polypropylene and in one instance (Example 39) isotactic polypropylene.